Photoresists capable of 0.1-0.2 μm resolution in production, using optical exposure, have already entered the market, whereas research efforts for materials and processes targeting at even lower dimensions are being explored. The exposure wavelength defines to a great extent the ultimate photoresist resolution properties. Currently, 248 nm and 193 nm exposures are mostly used for the production of 0.1-0.2 μm features and research efforts are underway for even smaller dimensions, using 157 nm and 13 nm electromagnetic radiation.
Photoresists used in modem lithographic applications should combine a number of properties in order to be suitable for high-resolution patterning. Among them the appropriate absorbance at the exposure wavelength and the resistance to the plasmas used during the pattern transfer step are especially important.
As the lateral structure dimensions shrink, thin photoresist films, usually below half micron, are used to keep aspect ratios low and thus to facilitate lithographic processes. Consequently, the importance of the film etch resistance for effective pattern transfer increases. Traditionally, the etch resistance properties of the resist material were obtained using phenolic polymers. This was the case in 436 nm, 365 nm and 248 nm exposure. Since aromatic polymers—novolacs as well as poly(hydroxystyrene) based resins—are highly absorbant to 193 nm light, single layer photoresist systems for 193 nm lithography are usually based on aliphatic polymers which show low absorbance, at this wavelength. However, aliphatic polymers have roughly twice as high etching rates than aromatic ones, classically used in previous generation photoresist systems (L. A Pederson, J. Electrochem. Soc., 129 (1), 205-8 (1982)), despite the initial reactivity of aromatic polymers in the plasma environment:
According to the accepted polymer etching mechanism, halogen atoms add to the double bonds first, as opposed to subtraction of hydrogen atoms from the saturated carbon bonds, and the resulting halogenated compound is less reactive than the product of abstraction (Tepermeister and H. H. Sawin, J. Vac. Sci. Technol. A 10 (5), 3149-57 (1992)). The etching rate was found to be inversely proportional to the [Carbon-Oxygen] atom content of the polymer (H. Gokan, S. Esho, Y. Ohnishi, J. Electrochem. Soc., 130 (1), 143-6 (1983)), and an empirical parameter referred to as The Ohnishi Parameter, O=(Total Number of Carbon atoms/(Carbon-Oxygen atoms) has been defined. The smaller the Ohnishi parameter (O) the higher the etch resistance. For example poly(hydroxystyrene)—a very etch resistant polymer—has O=2.43, while PMMA—a non-etch resistant polymer—has O=5. Etch resistance was also characterized with the amount of Carbon atoms present in a ring structure, and another improved empirical parameter referred to as The Ring Parameter R=(Mass of Carbon atoms in Rings)/(Total Mass of Polymer) was defined recently. The higher the value of R (and closer to unity), the higher the etch resistance of the polymer. See for example (R. R Kunz, S. C. Palmateer, A. R. Forte, R. D. Allen, G. M. Wallraff, R. A. DiPietro, D. C. Hofer, “Limits to etch resistance for 193-nm Single-Layer Resists”, Proceedings SPIE, Advances in Resist Technology and Processing, Vol. 2724, p.365-76, 1996). For example poly(hydroxystyrene)—a very etch resistant polymer—has R=0.6, while PMMA—a non-etch resistant polymer—has R=0. Notice that high etch resistance is characterized by high R-values (close to unity) and small O-values. In any case, high carbon and double bond content, high carbon content in ring structures, as well as low oxygen/nitrogen content are desirable for better plasma resistance. Thus, to address the etch resistance problem in 193 nm, polymeric materials containing cycloaliphatic moieties, either attached to the polymer chain or as separate components, were used by most resist companies and universities in an effort to obtain carbon-carbon bonding groups similar to the ones of the benzyl ring. Nevertheless, as it was first suggested (T. Naito, K. Asakawa, N. Shida, T. Ushirogouchi and M. Nakase, “Highly Transparent Chemically Amplified ArF Excimer Laser Resists by Absorption Band Shift for 193 nm Wavelength” Jpn. J. Appl. Phys., 33, 7028-32 (1994)), another route in order to enhance the etch resistance of these aliphatic polymers, while keeping acceptable absorbance, can be provided by polyaromatic compounds such as naphthalene, anthracene and their derivatives. These are characterized by a significant red shift of the absorption band, which in simple aromatic compounds is centered around 193 nm and thus, they are significantly more transparent at this wavelength. It has been shown by some of the inventors and collaborators (P. Argitis, M. Vasilopoulou, E. Gogolide s et al “Etch resistance enhancement”, Microelectron. Eng. 1998) that anthracene loading is effective in increasing the etch-resistance, i.e. reduction of the etching rate of the PMMA by more than 30% is obtained at loadings slightly higher than 5%. In comparison, addition of more than 10% w/w is required for the same etch resistance increase in the case of adamantane (cycloaliphatic) derivatives.
Similar challenges, i.e. to enhance etch resistance with polymers or additives that have suitable absorbance at the exposure wavelength, are encountered in other wavelengths, as well. For instance, in EUV (13 nm), one of the most probable spectral areas to be used in the next generation lithography, it has been recently shown, on the basis of absorbance considerations, that thickness of 0.15-0.25 μm could be tolerated for most materials. Since film thickness in this range is considered rather low compared to the thickness used so far, the highest possible etch resistance of the resist materials is desired. The increase of the resist etch resistance is also desirable even at longer wavelengths e.g. 248 nm or 436 nm, since it allows the use of thinner resist films and thus smaller aspect ratios for high resolution patterning. Analogous arguments can be also provided for 157 nm e-beam and X-ray lithographies.
The anthracene derivatives, mentioned above as possible etch resistance additives, have been also used in photoresist formulations to control absorbance and/or photochemical properties at certain wavelengths. Thus, 9-anthracene methanol has been used as near UV photosensitizer by a number of investigators (W. Conley and J. Gelorme, “Negative i-line resist for 0.5 μm and beyond”, J. Vac. Sci. Technol. B, 10 (1992) and U.S. Pat. Nos. 5,110,711, 5,098,816 and 5059512). On the other hand, other patents (see for example U.S. Pat. No. 5,731,125) have been granted for the use of anthracene derivatives, e.g. anthracene-9-carboxyethyl and 9-anthracene-methanol, as additives to control resist absorbance for chemically amplified resist compositions. Anthracene derivatives as deep UV absorbers have been also used in resist compositions described in U.S. Pat. No. 5,695,910. The absorbing properties of anthracene derivatives are also used in a number of recent patents (see for example U.S. Pat. No. 5,736,301) where anthracenes, incorporated in a polymer backbone or as additives are part of antireflective coatings placed under photoresist films. In this last case, where anthracene derivatives are used in the composition of antireflective coatings, special care is taken for the film to have optimum etch resistance. Too high etch resistance of the antreflective coating is undesirable, since it causes problems to the proper film removal after pattern transfer.
Other etch resistance additives, and resist compositions incorporating same are described in G. Dabbagh et al, Proceedings of the SPIE—The International Society for Optical Engineering; 3678 (1999) 86; European Patent Application EP-A-1126320; U.S. Patents U.S. Pat. No. 6,238,842 and U.S. Pat. No. 6,156,477; and German Patent Application DE-A-10009183, the contents of all of which are hereby incorporated by reference.
In general, the use of additives to improve certain properties in photoresist compositions is a versatile method, since it allows easy preparation of different material formulations, without requiring complicated polymer synthesis or chemical modification procedures. Nevertheless, restrictions are posed by the need to fulfill a number of physicochemical requirements defined by the lithographic process:
First, the additives must be compatible with the rest of the resist components to avoid any phase separation phenomena that can lead to film composition and property inhomogeneities.
Second, the additives should withstand the lithographic processing steps without decomposition, sublimation or process induced phase separation phenomena.
Finally, they should not modify substantially the resist chemical and physical properties leading to deterioration of the lithographic performance.